The present invention relates to vibrators used in seismic exploration, and, in particular, to a system and method for seismic exploration utilizing a hydraulically actuated three-axis vibrator.
Seismic exploration typically involves the introduction of shock waves, or vibrations, into a geologic formation. The reflected portion of shock waves passing through differing strata in the formation are then monitored. The reflected shock waves are used to estimate the shape, composition, and depth of various strata within a formation. The predominant method of inducing these vibrations is by hydraulic actuators of a variety of forms. U.S. Pat. No. 5,401,919 to Crowell et al., the contents of which are incorporated by reference, describes one such form for generating compression waves.
Patents illustrating three-axis-capable seismi c vibrators include U.S. Pat. No. 4,662,473 to Betz; U.S. Pat. No. 4,665,314 issued to Airhart; U.S. Pat. No. 4,660,675 issued to Airhart; U.S. Pat. No. 4,719,607 issued to Airhart. Each of these patents illustrates an apparatus capable of producing vibratory seismic waves at varying inclinations and azimuths without repositioning the ground engaging base plate of the apparatus or without repositioning the vehicle which carries such apparatus. However, these prior art devices require repositioning or reconfiguration of some component, for example, the vibratory mass, in order to generate both orthogonal shear waves and compression waves. Additionally, these devices can not produce shear waves without also generating a significant compression wave at the same time.
U.S. Pat. No. 4,853,907 describes an inclinable seismic vibrator. The apparatus provides a means "for imparting vibratory signals to the earth along selected vector paths." The apparatus is mountable to a vehicle frame and includes a ground-engaging base plate 18 which connects to a lower lift frame 14. Col. 3, lines 64-65. The vibrator 34 includes a universal joint 38 which connects a shaft 36 to the base plate 18. Col. 4, lines 18-22. A vibratory mass 44 mounts on the shaft 36, and reciprocates the shaft to impart vibrations into the baseplate 18. Col. 4, lines 30-31.
The prior art does not provide a device for generating seismic vibrations along three axes simultaneously, nor can it selectively impart shear wave only. Therefore, what is needed is a seismic vibrator which simultaneously or selectively imparts compression waves (P-waves) and orthogonal shear waves (S1- and S2-waves) into the Earth's surface. In addition, what is needed is a system and method which more precisely, and in a more shock-isolating arrangement, controls the inclination and the azimuth of the seismic source.
Another prior art P-wave isolation scheme uses vertical chains to maintain the baseplate and a foot in a proper orientation when the baseplate is in an up and stored position. In addition, isolation air bags act vertically while an assortment of restraining linkages rigidly restrain the vibrator against transverse motion. However, in combined P-wave and S-wave operation, rigid transverse restraint would permit a large portion of the S-wave energy to transfer to the vehicle. Therefore, to produce S-waves and provide for greater shock isolation, the output energy which transfers to the vehicle must be minimized. At the same time, the selection of a transverse restraint system having an excessively weak restraint would limit the seismic vibrator to operation on terrain which is level and flat. This is because use on a slope or incline would permit the vibrator to move laterally into the vehicle frame during operation.
The solution to the above-identified problem, which is inherent in a three-axis vibratory system, is not addressed in the prior art. Therefore, what is needed is a vibration isolation scheme which provides an appropriate compliant restraint of the seismic vibrator evenly along multiple axes.